Download Segment polarity genes Defining A/P axis within a segment Figure

Figure 5.2
Segment polarity genes
Defining A/P axis within a segment
Hierarchy of gene
expression
compartmentalizes
embryo along A/P
axis
Figure 5.25
Pair-rule genes
(fushi tarazu and
even-skipped)
define parasegments
Figure 5.24: engrailed is expressed in the
anterior region of each parasegment
•Lineage restriction
•Expressed through life of fly
Engrailed
expression is
controlled by the
action of these
genes
1
In absence
of ftz,
engrailed
is
expressed
only in odd
stripes
Expression
of pair-rule
genes
defines
expression
of segment
polarity
genes
Wingless
repressed by eve
and ftz
Engrailed activated by
eve and ftz
Figure 5.29
•feedback between engrailed
and wingless maintains and
reinforces initial pattern
•Induction of engrailed and
hedgehog by wingless
Figure 5.28: expression of segment polarity genes
in parasegments (pre-gastrulation) and segments
•Induction of wingless by
hedgehog
--> sharp boundaries
2
Figure 5.29
If wingless is mutated,
no hedgehog or
engrailed are expressed
No parasegment
boundaries
(See Figure 5-28)
Patched can suppress wg expression, but if Hh binds to smo,
that suppression is inhibited
Gradients of
hedgehog
and wingless
define
regions
within the
segments
3
The parasegments are examples of “compartments” regions of the organism that are lineage restricted
Assessing
compartments
by somatic
clones
Box 5B
•Mitosis generates
clones from the
different daughter
cells
•Heterozygous
individual
Box5B
•X-rays
induce
mitotic recombination
Figure 5.26:
Clones mutant for minute
divide more rapidly than
normal cells
engrailed defines A/P
compartments throughout
development
Establishing A/P
axis
Defining regions
more precisely
Defining parasegments and
their orientation
Defining segment
identity
4
Figure 5.2
Selector genes determine
segment identity
Combination of
gap gene and pairrule gene
expression
patterns define the
selector gene
expression pattern
Box 4A: 3’--> 5’ organization reflects anterior--->
posterior expression
Figure 5.35: selector genes are arranged in two complexes
5
Figure 5.36
Mutations in selector genes
result in homeotic
transformations - one body
part is transformed into
another
The dominant
Antennapedia mutation
causes legs to develop in
place of antenna
(gain-of-function - gene is
expressed ectopically in the
head as well as the thorax
recessive mutation - (loss-offunction) gene is not
expressed in thorax, antenna
are formed in place of legs
Initial expression of the
gap gene Krüppel predicts
later expression of
selector gene
Antennapedia
Box 4A: 3’--> 5’ organization reflects anterior--->
posterior expression
6
Figure 5.37
Expression of the 3 genes
in the bithorax complex
defines parasegment
identity
If delete all 3 genes of
the bithorax compex,
all segments
transformed to
segment 4
Ubx gene
has a
number of
regulatory
regions
that
control
expression
in a
particular
region
If add Ubx back, get 4,
5 and 6…
•Ubx specifies
parasegments 5 and
6
Figure 5.37
7
If Ab-A is also restored,
identity of parasegments 7
and 8 is restored (all
remaining like parasegment
9)
Figure 5.37
Expression of selector
genes defines
parasegment identity
If Ab-A and Ab-B are
restored, identity of all
parasegments except 5 and 6
are restored
Expression of anterior homeotic genes is
repressed by posterior homeotic genes
Chromatin remodeling maintains selector
gene expression in absence of gap genes
“posterior dominance”
Polycomb - maintains repression
Trithorax - maintains activation
8
Establishing A/P
axis
Defining regions
more precisely
Defining parasegments and
their orientation
Defining segment
identity
9